A single quadrupole mass spectrometer introduces, into a quadrupole mass filter, various ions generated from a sample, allows only ions having a specified mass-to-charge ratio to selectively pass, and detects the ions that have passed by using a detector to acquire an intensity signal according to an amount of ions.
Generally, a quadrupole mass filter includes four rod electrodes disposed in parallel with each other to surround an ion optical axis. A voltage obtained by adding a direct current voltage and a radio-frequency voltage (alternating current voltage) is applied to each of the four rod electrodes. The mass-to-charge ratio of ions that can pass through the space surrounded by the four rod electrodes in the axial direction of the space depends on the radio-frequency voltage and the direct current voltage applied to the rod electrodes. Therefore, by appropriately setting the radio-frequency voltage and the direct current voltage according to the mass-to-charge ratio of ions to be measured, it is possible to pass the ions to be measured, and to detect the ions. In addition, by changing the radio-frequency voltage and the direct current voltage to be applied to the rod electrodes while maintaining a predetermined relationship between them within a predetermined range, it is possible to scan the mass-to-charge ratio of the ions passing through the quadrupole mass filter in a predetermined range and to create a mass spectrum on a basis of signals obtained from the detector.
The operating conditions in which ions pass stably and the behavior of ions in a quadrupole electric field generated in the space surrounded by the rod electrodes by the voltage applied to the rod electrodes constituting the quadrupole mass filter are conventionally analyzed in detail, as described in Non Patent Literature 1 and other literatures.
That is, movement of ions passing through an ideal quadrupole electric field generated in the space surrounded by the rod electrodes extending in the z axis is represented by the following Equations (1) which are called the Mathieu equations.m(d2x/dt2)=−(2zex/r02)(U−V cos Ωt)m(d2y/dt2)=−(2zey/r02)(U−V cos Ωt)  (1)Here, m is the mass of an ion, r0 is the radius of the circle inscribing the rod electrodes, e is the electric charge, U is the direct current voltage value, V is the amplitude of the radio-frequency voltage, and Ω is the frequency of the radio-frequency voltage. Further, z represents a position on the z axis, and x and y respectively represent positions on the x axis and the y axis which are both orthogonal to the z axis.
Conditions that ions can pass stably within the space surrounded by the four rod electrodes can be demonstrated by the following Equations (2) which represent a region on a two-dimensional space with the following two parameters a and q obtained by solving the Matthew equation set as axes orthogonal to each other.ax=−ay=8 eU/mr02Ω2 qx=−qy=4 eV/mr02Ω2  (2)
FIG. 11 (a) is the stable state diagram often used to describe a stability condition for a solution to the Matthew equation. In FIG. 11 (a), the nearly triangular region surrounded by the solid lines is the stable region represented by the stable solution of Equation (1), and the outside of the triangular region is an unstable region in which ions disperse. Theoretically, ions having a certain mass can pass stably if conditions including voltage are set so that the ions are positioned anywhere within the stable region. But, in order to obtain high mass resolution, it is necessary to set the operating conditions at a position close to the top P of the stable region. Therefore, in general, the operating conditions are determined, for example, near a point A close to the top P in order to maintain high mass resolution and to prevent the ions from entering the unstable region even if the operating conditions deviate or fluctuate.
However, in actual measurement by the quadrupole mass spectrometer, ions generated outside the quadrupole mass filter enter the space surrounded by the rod electrodes via an end (entrance) part of the space. The electric field at the end part, that is, an edge end electric field, is weaker than the quadrupole electric field generated within the space. Therefore, behavior of the ions entering the quadrupole mass filter, which is caused by the electric field experienced by the ions, can be shown on the stable state diagram by the dotted line arrow in FIG. 11 (a), that is, the ions enter the stable region after passing through the unstable region. Since movement of the ions is unstable while passing through the unstable region B in the diagram, part of the ions disperse and disappear before reaching the stable quadrupole electric field. This is a major factor of a decrease in the passing efficiency of ions passing through the quadrupole mass filter.
In order to solve the above-described problem in many quadrupole mass spectrometers, the quadrupole mass filter employs a configuration in which, just in front of a main electrode section composed of four main rod electrodes for selecting ions according to the mass-to-charge ratio, a pre-electrode section composed of four pre-rod electrodes having the same diameter as that of the main rod electrode and a length shorter than that of the main rod electrodes is disposed, and the same radio-frequency voltage as that applied to the main rod electrodes is applied to the pre-rod electrodes (see Patent Literatures 1 and 2, Non Patent Literature 2, and other literatures). The direct current voltage applied to the main rod electrodes for ion selection is not applied to this pre-rod electrodes. Therefore, as described in Patent Literature 2, behavior of the ions first passing through the space surrounded by the pre-rod electrodes and then entering the space surrounded by the main rod electrodes can be shown on the stable state diagram by the dotted line arrow in FIG. 11 (b), that is, ions reach the point A while passing through the stable region. In this case, since the ions do not pass through the unstable region, the ions are efficiently introduced into the space surrounded by the main rod electrodes, and ion passing efficiency can be improved compared with a case where the pre-rod electrodes are not provided.
However, according to the present inventors' simulation calculations and other studies, even for the above-described quadrupole mass filter provided with the pre-rod electrodes, most of the ions entering the quadrupole mass filter are not used, and still much improvement should be done in the ion passing efficiency. In recent years, in the field of mass spectrometry, identification and measurement of components contained in a sample in very small quantities are becoming increasingly necessary. In order to meet such a request, further improvement in detection sensitivity is necessary. For the quadrupole mass spectrometer equipped with the quadrupole mass filter, it is very important to improve the ion passing efficiency of the quadrupole mass filter.